The present invention relates to a device for degassing melts, in particular plastic melts, comprising a melt nozzle which has an inlet connection for connecting to a melt supply and a melt outlet opening, as well as a degassing chamber defined by essentially gas-impermeable walls, communicating with a vacuum source and opening into a discharge opening for essentially tight connection to a melt processing system, the melt outlet opening of the nozzle extending into the degassing chamber.
Such a device for degassing plastic melts is known from an extruder system. In this, granular plastic is molten under heat and pressure in an extruder, and this melt is discharged from the extruder. However, the melt is not homogenous, but contains air bubbles, which make further processing of the melt difficult. For this reason the melt has to be degassed, i.e. freed from air and other gas bubbles. This can be achieved most easily by connecting a vacuum portion to the extruder discharge opening, through which vacuum portion the melt flows. Because of the negative pressure in the vacuum portion, gas bubbles are drawn out of the melt and let off. Because of the high viscosity of the melt this process only works quite inadequately and furthermore necessitates a high vacuum so as to make it possible to draw gas bubbles even out of the central region of the melt. High-vacuum apparatus are very expensive and sensitive, however, and need frequent maintenance. Besides, the gases drawn out are hot (typically 250xc2x0 C. to 260xc2x0 C.), and they often contain aggressive vaporized plastic additives that attack the apparatus so that it may be necessary to replace it several times a year.
Thus, in order to be able to reduce the necessity to employ high vacuum, a device of the above type, as taught in U.S. Pat. No. 3,410,938 or the corresponding CH Patent No. 425 207 A, has been developed, wherein the melt flow initially flows through a nozzle connected to the extruder outlet, which nozzle is provided with a nozzle plate having a plurality of through-holes or slits dividing the melt flow into a plurality of xe2x80x9cspaghetti-likexe2x80x9d or ribbon-like sub-streams. A degassing chamber is connected downstream of the nozzle plate, which is under vacuum and through which the melt sub-streams flow after leaving the nozzle. As the diameter of the melt sub-streams is smaller than that of the flow as a whole it is not necessary to employ a vacuum as high as that employed for directly degassing the melt flow as a whole. However, practical use of this device also has disadvantages, as research carried out by the inventors of the subject matter of the present application has resulted in the discovery that the melt sub-streams have a strong tendency of adhering to each other after leaving the nozzle plate, thus again combining to form a thicker stream, so that the vacuum employed will not be enough to draw the gas bubbles out of the interior of the melt.
The present invention is based on this discovery and offers a solution for degassing melts in a satisfying way that avoids the disadvantages of the state of the art, while at the same time making it possible to further reduce the vacuum required. The invention emanates from the approach that dividing the melt into sub-streams cannot bring about the desired result as the subsequent reunification of the sub-streams can only be prevented at unduly great efforts. Thus the invention continues to employ an undivided melt flow, which is, however, brought in a shape having a cross-section of very small thickness, by forming the melt outlet opening of the nozzle as a single slit whose length is a multiple of its width and which has a curved or folded configuration in order to accommodate the required length of the slit in the nozzle plate. This measure according to the invention makes it possible to degas the melt to a satisfying degree already at very slight vacuum, for the gas bubbles only have to be drawn to the surface of the melt from a very small depth thereof or even burst by themselves because of the small thickness of the melt flow.
Conveniently the slit has a length that is at least 50 times, preferably at least 500 times, larger than its width. In some applications it may even be considered appropriate to increase the ratio between length and width to 5000:1 and above.
In order to further reduce the amount of vacuum necessary it may be provided for the width of the slit to be less than 2 mm, preferably less than 0.1 mm.
In order not to limit the throughput of melt from the melt supply, e.g. from an extruder, to the subsequent processing means by the interposed degassing device, the cross-sectional area of the slit may not be smaller than a predetermined value. As the width of the slit may not be increased without negatively affecting the degassing performance of the device according to the invention, it is thus necessary to adjust the length of the slit to a suitable value. In order to do so without at the same time unduly increasing the total size of the nozzle, an embodiment of the invention is characterized in that the slit is formed in zigzag or corrugated form in its longitudinal direction.
A particularly favorable embodiment of the device according to the invention is obtained by forming the slit as a ring, as in this case the nozzle may be produced by turning it on a lathe and its total dimensions may be kept small. If it is necessary to further increase the length of the slit, it is of course also possible to form the annular slit in corrugated or zigzag configuration.
In order to avoid the disadvantage that a vacuum is only applied to the exterior surface of the melt hose thus produced while it flows through the degassing chamber, a particularly preferred embodiment of the invention is characterized in that the nozzle has a through-hole extending into the interior of the ring and communicating with a vacuum source. By this measure a vacuum is applied to the melt hose both from the outside and the inside at the same time, which because of the small wall thickness of the melt hose ensures high efficiency of the degassing device according to the invention. The construction of the device is simplified if the through-hole of the nozzle opening into the interior of the ring of the slit communicates with the same vacuum source as the degassing chamber.
In order to be able to vary the thickness of the melt leaving the slit in accordance with the intended use and material properties of the melt, the invention furthermore provides for the nozzle to be equipped with replaceable inserts for slits of different shapes and/or dimensions.
Providing the nozzle and/or the degassing chamber with heating means makes it easier for the melt to flow through the degassing device.
In order to monitor the degassing process, at least one viewing window may be provided in the walls defining the degassing chamber.
Preferably a filter for filtering foreign matters out of the melt flow is provided at the inlet connection of the nozzle. Thus is it possible to prevent foreign matters from entering the degassing device, which would be in particular harmful if they were inflammable at the temperatures in the degassing device, as undesirable combustion gases would be formed during their combustion.
According to a particularly advantageous embodiment the device according to the invention comprises a Venturi fluid entrainment pump as its vacuum source. Such a pump does not comprise any movable parts but produces a negative pressure in a Venturi tube by a fluid flowing therethrough, which negative pressure sucks fluid from a side entrance into the tube, i.e. in the present case the gases to be withdrawn from the degassing chamber. The Venturi tube may be provided easily with a corrosion-resistant coating on the inside, so that it withstands the aggressive components of the gas to be withdrawn.